Enantiomeric Separations in Capillary Electrochromatography with Crown Ether-Capped -Cyclodextrin-Bonded Silica Particles as Chiral Stationary Phase

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Two novel types of crown ether capped b-cyclodextrin (b-CD) bonded silica, namely, 4'-aminobenzo-Xcrown-Y (X 15, 18 and Y 5, 6, resp.) capped [3-(2-O-b-cyclodextrin)-2-hydroxypropoxy] propylsilylappended silica, have been prepared and used as stationary phases in capillary electrochromatography (CEC) to separate chiral compounds. The two stationary phases have a chiral selector with two recognition sites: crown ether and b-CD. They exhibit excellent enantioselectivity in CEC for a wide range of compounds. After inclusion of metal ions (Na or K ) from the running buffer into the crown ether units, the stationary phases become positively charged and can provide extra electrostatic interaction with ionizable solutes and enhance the dipolar interaction with polar neutral solutes. This enhances the host-guest interaction with the solute and improves chiral recognition and enantioselectivity. Due to the cooperation of the anchored b-CD and the crown ether, this kind of crown ether capped b-CD bonded phase shows better enantioselectivity than either b-CD-or crown ether bonded phases only. These new types of stationary phases have good potential for fast chiral separation with CEC.Introduction. ± Separation of enantiomers is one of the most active areas of chromatography [1 ± 3] and is important in various fields [4 ± 6], e.g., natural-product research, stereospecific synthesis, development of chiral drugs in the pharmaceutical industry, and in environmental studies. Chromatographic methods are typically employed for chiral separation, including high-performance liquid chromatography (HPLC) [7], gas chromatography (GC) [8], micellar electrokinetic capillary chromatography (MECC) [9] and, more recently, capillary electrochromatography (CEC) [10]. To improve chiral separation, two approaches can be used: a) optimization of chromatographic conditions, and b) use of new chiral stationary phases (CSPs) or chiral additives in the mobile phase.GC and LC were the first tools to be employed in the separation of enantiomers [4]. However, conventional LC cannot utilize small particles and/or long columns to obtain high efficiencies because of the pressure limitations of normal pumping systems. Although GC usually offers higher efficiencies than LC due to the larger number of available theoretical plates, most chiral compounds of interest have low volatilities. This limits the use of GC for chiral separations [10]. During the last several years, capillary electrophoresis (CE) has become a powerful technique for chiral separations when chiral selectors are added to the running buffers [11]. CEC is a modern liquid chromatography technique combining the high efficiency of CE with the high selectivity usually obtained by HPLC [2]. The mobile phase is transported through a Helvetica Chimica Acta ± Vol. 85 (2002) 3283 capillary containing the stationary phase by means of electro-osmosis instead of pressure. Neutral solutes are separated by partitioning between the...

Chiral Separation of Basic Compounds on Sulfated β-Cyclodextrin-Coated Zirconia Monolith by Capillary Electrochromatography

“…This is favorably compared with the separation of NAD on a chiral stationary phase incorporating multiple chiral-selectors such as crown ether-capped β-cyclodextrin bonded silica where only two peaks were obtained. 44 No appreciable decline in resolution and retention after over fifty injections, and run-to-run and day-to-day repeatability of the column of less than 3% indicate that the SCD-ZM column is practically stable in the eluents with added SCD.…”

Application of Cyclam-Capped β-Cyclodextrin-Bonded Silica Particles as a Chiral Stationary Phase in Capillary Electrochromatography for Enantiomeric Separations

“…The bromoacetate-substituted-(3-(2- O -β-cyclodextrin)-2-hydroxypropoxy)-propylsilyl-appended silicaparticles (BACD-HPS), the intermediate product for the synthesis of crown ether and cyclam-capped CD-bonded silica particles, have been previously packed into fused-silica tubing to fabricate 75-μm-i.d. × 30-cm effective length columns (38.5 cm total length, same as DCCD-HPS- and MCCD-HPS-packed columns) and used in CEC for comparative studies . Under the same separation conditions (ACN/Tris−HCl (50:50), 10 kV), for the enantiomers of 1-phenyl-2-propanol, only partial enantioseparation (α = 1.08, R S = 0.63) on the BACD-HPS-packed column was achieved; however, baseline separation (α = 1.25, R S = 2.19) was achieved on the MCCD-HPS-packed column.…”

“…× 30-cm effective length columns (38.5 cm total length, same as DCCD-HPS- and MCCD-HPS-packed columns) and used in CEC for comparative studies . Under the same separation conditions (ACN/Tris−HCl (50:50), 10 kV), for the enantiomers of 1-phenyl-2-propanol, only partial enantioseparation (α = 1.08, R S = 0.63) on the BACD-HPS-packed column was achieved; however, baseline separation (α = 1.25, R S = 2.19) was achieved on the MCCD-HPS-packed column. Greater retention was obtained on the MCCD-HPS-packed column ( k 1 ‘ = 0.65, V EOF = 0.27 mm s -1 ) than on the BACD-HPS-packed column ( k 1 ‘ = 0.47, V EOF = 0.28 mm s -1 ).…”

Microwave-Assisted Preparation of a β-Cyclodextrin-Based Stationary Phase for Open Tubular Capillary Electrochromatography

“…Bromoacetyl bromide (0.05 mL) in DMF (10 mL) was added dropwise. The resulting mixture was stirred for 1 h at 0-5 C and then for 4 h at 25 C. The DMF and excess bromoacetyl bromide were removed by evaporation under reduced pressure (Gong and Lee 2002;Ni et al 2002).…”